A gaseous or solid additive must be thoroughly contacted with a metal melt in many metallurgical processes. German patent document No. 2,209,902 filed Mar. 1, 1972 by H. Richter et al describes a steel-refining process wherein calcium or calcium compounds such as CaO, CaF.sub.2, Ca-Si, CaC.sub.2 are blown in an inert vehicle gas into the melt. Calcium at refining temperatures of about 1600.degree. C. has a vapor pressure of 2.13 atm, so that below a depth of 1.7 m this element is a liquid. The lance through which this solids/gas suspension is introduced thus extends down some 2 m below the melt level. The calcium droplets rise slowly and are extremely well contacted with the steel for efficient material use.
The lance has an extremely short service life. The conditions of heat and corrosive chemicals quickly destroy it. In addition, even in procedures other than desulfurizing, this lance is inconvenient and in the way.
It is also known to increase the carbon content of a steel melt, either to improve its quality or to generate heat by subsequent oxidation, by several techniques. Carbon can be introduced by lances in an inert carrier gas and/or a carbon-containing gas can be supplied to crack in the melt and release carbon. Since carbon-containing gases normally also contain hydrogen, which cannot be permitted in steel, this latter procedure is rarely used. It is also possible, as suggested in German patent document No. 2,838,983 filed Sept. 7, 1978 by K. Brotzmann, to treat the melt by means of an oxygen jet played over the melt surface and carrying carbon or appropriate carbon compounds, or even to form such jets by nozzles immediately below the melt surface.
Another procedure uses nozzles provided on the floor of the vessel, and through which the additives are blown. Such nozzles must be made of very valuable material to last at least the life of the oven lining, so that they are very expensive. Furthermore something, if only an inert gas, must be fed in through them all the time to prevent the melt from entering and clogging them, as it is not necessary to continue the additive introduction through the entire life of the melt. Such use of gas can consume important quantities of these gases, which are not necessarily cheap.
Commonly owned Luxembourg Pat. Nos. 82,552, 82,553, 82,554, issued Jan. 20, 1982 and 82,597 issued Feb. 17, 1982 describe a ceramic body forming part of the base of a metallurgical crucible. This sintered ceramic has passages of a small size such that the treatment gas can be introduced while the high surface tension of the melt prevents the melt from flowing back into the openings of the body to block them. Thus this insert remains gas-pervious even if a gas is not forced continuously through it.